Small-size toners having a particle size of not larger than 9 .mu.m have been used in recent years to provide an image of higher quality. An image of higher quality can be provided by making toner particles finer, but there are caused problems that transferability (i.e., transferring property) and cleaning properties are lowered. To solve these problems, various additives such as transfer aid and cleaning aid have been added to the toner particles. For example, an attempt to improve the powder fluidity of toners has been made by adding fine silica particles to thereby improve transferability, or an attempt to improve cleaning properties has been made by adding lubricants or polymer beads.
These problems can also be improved by lowering the chargeability (i.e., tribocharge) of the toner. In this case, however, there is posed a problem that the occurrence of toner cloud is marked and a trouble due to contamination in machines is caused. FIG. I-1 and FIG. I-2 illustrate these phenomenons. FIG. I-1 is a graph illustrating the relationship between the charge quantity of a toner and adhesion (i.e., adhesive power). FIG. I-2 is a graph illustrating the relationship between the charge quantity of a toner and a toner-peeling electric field (an electric field required for peeling off the toner). It can be seen from these figures that in the case where the charge quantities of the toners are the same, the adhesion of the toner is independent of particle size, while the electric field for peeling off the toner is greatly increased with a reduction in the particle size of the toner. This means that the latitude of transfer cloud becomes greatly narrow by making the particle size of the toner finer.
When external additives are conventionally added to and mixed with the toners, the toners themselves have a particle size of as large as 10 .mu.m or more and hence transferability is good. Accordingly, only a fluidizing agent conventionally is added to and mixed with the toners. In those toner compositions, the fluidizing agent is mixed so as to allow it to firmly adhere to the toners. The addition of two types of external additives having a particle size of not larger than 30 .mu.m to toner particles having an average particle size of 12 .mu.m has been proposed to increase the charge quantity and at the same time to improve transferability as disclosed in JP-A-2-151872 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). In such a toner composition, the external additives are mixed under such conditions that the additives are allowed to firmly adhere to the toner particles. In carrying out transfer, the particle size of toner greatly contributes to transferability and a contact area between a photoreceptor and the toner is small so that transfer can be relatively well-made irrespective of the mixing conditions.
However, when small-size toners are used, the charge quantity per gram is increased and transferability is lowered. Hence, the particle sizes of the additive particles should be large and as a result, there is a problem that good transferability can not be obtained depending on mixing conditions.
Further, when small-size toners are used, surface area per gram is increased. Accordingly, a large amount of a fluidizing agent is used and mixing must be conducted so as to allow the fluidizing agent to firmly adhere to the toner to obtain fluidity.
Furthermore, when fine particles having a large particle size and fine particles having a small particle size are added to the toner, there are caused problems that the mixing of the particles having a large particle size is insufficient under conventional addition and mixing conditions for fine particles having a small particle size, a toner to which the external additives uniformly adhere can not be obtained and transferability is insufficient. FIG. II-1(B) is a sectional view of a toner where fine particles having a large particle size non-uniformly adhere to the toner, and the fine particles 2 having a large particle size concentratedly adhere to recessed areas (i.e., concave portions) on the surface of toner particle 1.
Accordingly, it has been demanded to improve developing properties, transferability (i.e., transferring property) and cleaning properties without causing the occurrence of toner cloud in development, namely, without causing a lowering in the charge quantity of the toner. The present invention has been accomplished to meet such requirements.